The present invention relates to an offset amount measuring apparatus suitable for use in measuring an offset amount in reproduced signals resulting from an offset between the center of the circular data track of a disk and the center of rotation of the disk.
Conventionally the measurement of an offset amount produced at the time the disk having circular data track was chucked on the revolving shaft of the spindle motor has been carried out with the use of a precision rotating stage and a microscope.
Such an offset amount measuring method is difficult to be practiced in a small disk drive apparatus.
There is also disclosed a method to correct the offset of the disk produced when it is chucked, in Japanese Patent Laid-open No. Sho 63-61478 (laid open on 1988.03.17). This method utilizes a dedicated area provided on the disk to form therein a position signal pattern to measure an offset amount, the offset amount is measured from a reproduced signal of the pattern, and the offset amount measured is used for offset correction.
In such a case, however, it is required to provide a dedicated area on the disk for the measurement of the offset amount and the dedicated area has to have a sufficient margin in order that the position signal for the offset amount measurement is read unaffected by the offset.
Hence, there arises a problem that the data recording capacity of the disk is decreased accordingly.
Further, sometimes noise is produced superposed on the reproduced signal of the position signal pattern, and hence a problem arises that the accuracy of the detected offset amount is impaired.